This invention is related to optical spectroscopy, specifically to a lock-in optical spectrometer system.
Optical spectroscopy is a pervasive tool across physical, chemical and biological sciences. In a typical spectrometer, the light focused on the entrance slit is dispersed by a diffraction grating which spatially separates the light according to wavelength. Spectroscopic information is obtained by scanning the angle of the diffraction grating, and allowing different wavelengths to pass through an exit slit. The availability of photodetectors, such as one dimensional charge-coupled device (CCD) and two dimensional focal plane array (FPA) detectors has greatly reduced the amount of time required for the acquisition of optical spectra. Further parallelization has been achieved by using so-called “imaging” spectrometers (or spectrometers) that are optimized to preserve the vertical point-spread function of the spectrometer, allowing multiple input fibers to be imaged onto different horizontal portions of the detector array without interference. The increased throughput of such parallel techniques has become critical for applications such as Raman spectroscopy and near-field scanning optical microscopy, where the signals of interest are small and background-free.
For many applications, the signal of interest is superimposed upon a large noisy background. Applications that fit this description include spectrophotometry, and a wide variety of linear and nonlinear spectroscopies in which information about the sample is encoded in the intensity or polarization of light. A simple background subtraction can be achieved by synchronizing the frame acquisition to an optical chopper or similar modulator. However, the spectral response of such a technique is functionally equivalent to a comb filter and thus is less effective than bandwidth narrowing methods that use true lock-in amplification. Furthermore, the range of frequencies available is limited by the frame rate which is usually quite low compared to the frequencies at which one would like to operate. Laser noise usually falls off significantly above a few kHz, higher than the rates of most high-sensitivity photometric imaging devices such as charge-coupled devices (CCDs) and focal-plane arrays (FPAs). As a consequence, phase-sensitive detection or lock-in amplification is the method of choice in the above mentioned situations. However, due to the high photon fluxes and large backgrounds, it is not straightforward to parallelize lock-in techniques.
A straightforward implementation of a lock-in optical spectrometer (LIOS) requires a one-dimensional detector array and a minimum of one lock-in channel per detector. Lock-in amplifiers that employ digital signal processing (DSP) methods have rapidly replaced the original analog lock-in amplifier design. Digital lock-ins have superior phase stability and dynamic range, making them favorable for most demanding applications except perhaps mili-Kelvin transport experiments where digital circuitry is usually avoided. Banks of up to 32 digital lock-in channels are commercially available (http://www.signalrecovery.com), but for a reasonable number of channels (e.g., 256 or 512), this approach quickly becomes financially burdensome and unwieldy.